Cryptography
(advanced extra curricular topic)
 Cryptography (from the Greek words Kryptos, meaning
hidden, and graphein, meaning to write) is the study of
sending and receiving secret messages.
 The message to be sent is called plaintext. The disguised
message is called ciphertext.
 The process of converting from plaintext to ciphertext is
called encryption, while the reverse process of changing
from ciphertext back to plaintext is called decryption.
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Cryptography
The function used in the process of encryption and
decryption is called an encryption function.
The encryption function f is called the encryption
key and f −1 is called the decryption key.
Ideally, only the sender and the receiver know these
two keys.
If f is known then f −1 is known, so there is only one
key and both the sender and the receiver have this
key.
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Cryptography
 RSA Cryptosystem
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Cryptography
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Cryptography
RSA Cryptosystem
This pair is the public key for B and B that keeps the
pair (n, d) = (8633, 1207) secret. Notice that B does
not make public the prime numbers p, q and also
keeps the pair (n, d) = (8633, 1207) secret.
This pair (n, d) is the decryption key for B, and the
pair (n, k) is the encryption key for anyone who
wants to send the message to B. A will encrypt the
message by using this encryption key.
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Mathematical Foundations of
Cryptography
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Cryptography
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